High Luminosity Electron-Positron Higgs Boson Factory in LHC Tunnel

LEP3: A high luminosity electron-positron Higgs boson factory in the LHC tunnel A possible back-up to the preferred option (FCC-ee and FCC-hh) for the next accelerator for CERN In the LHC Tunnel 1 ESPP Venice Jun25

Layout of Talk Introduction Technical Parameters and key issues R&D needs in general are similar to those for FCC-ee. Two are of particular relevance for LEP3 high power fundamental power couplers, nested superconducting quadrupoles/sextupoles. Plus dedicated studies (see later) Physics running scenarios: (centre-of-mass energies, luminosities, ..); Resource requirements (capital cost for construction of the accelerator and the detectors) Annual cost of operation at each stage, human resources; Environmental impact (peak (MW) and integrated energy consumption: 250 MW at ZH and 185 MW at Z), ~TWh/yr during running. CO2 equivalent cost of construction much lower cost as little construction is envisaged Timeline (technically limited? Start data-taking ~ 5 years after HL-LHC stop; time for approval/feasibility? TDRs by mid-2030 s). Physics potential - will also be compared by Physics Preparatory Groups (PPG) 2 ESPP Venice Jun25

Introduction: Possibilities for PPs Future Long-Term Aim in particle physics: an accelerator with ~10 times higher than LHC (1-2 ~10 TeV). If FCC(ee) or CEPC is built then a ~100 TeV hadron collider would be the obvious next step. Muon colliders may provide alternative approach to reach constituent com energies of 10 TeV or higher. Programme could have milestones along the way that would deliver physics as the R&D progresses - muon driven neutrino beams for experiments such as nuSTORM or those on neutrino/antineutrino factories.) 3 ESPP Venice Jun25

Setting the Scene 2: ESPP Request for Alternatives We support FCC-ee and FCC-hh as the preferred option for CERN s future, as it addresses two key recommendations from ESPP2019. However, the ESG's remit explicitly states that The Strategy update should include the preferred option for the next collider at CERN and prioritised alternative options to be pursued if the chosen preferred plan turns out not to be feasible or competitive . In this context we take not feasible to mean not affordable . So, by definition, any alternative plan would have to represent lower cost while allowing simultaneous pursuit of the long-term ~10 TeV machine. We propose reusing the existing LHC tunnel for an electron-positron collider, called LEP3, as a back-up/alternative if the FCC cannot proceed. 4 ESPP Venice Jun25

LEP3: Key Issues This is not a new proposal! Several others have also discussed such a possibility in the past. e.g. 2013 ESPP, a Higgs factory (LEP3) was proposed but not pursued any further. [https://cds.cern.ch/record/1471486]. During the last 9 mo. progress has been made on several fronts, mostly using work done for FCC-ee. It is clear further progress needs to be made to solidify the proposal. What instantaneous luminosity is achievable? LEP3 lattice: a preliminary LEP3 lattice was proposed in 2017 (K.Oide & D. Shatilov). But high power-consumption requires superconducting quadrupoles/sextupoles What RF cavities should be used? Much work has been done by the CERN RF group. We have adopted their proposal. What are the implications of introducing a crossing angle (30 mrad)? We have examined the necessity of civil engineering works. We aim to limit excursion of beams and attain their crossing at the centre of existing caverns. Needs further study. What is the cost? We have used FCC-ee costs and scaled according to the number of components needed. The RF costs have been given by the CERN RF Group. Cost to the community can be contained by reusing ATLAS and CMS experiments 5

e+eColliders: Instantaneous Luminosity & s In circular e+e- colliders, for the same synchrotron power loss, at a lower energy, more current can be put to increase luminosity. LEP3: Run at s =230 GeV Only 10% higher energy than LEP. 18% lower synchrotron power loss at s=230 GeV compared with 240 GeV. 6 ESPP Venice Jun25

Follow ESPP2013 proposal & FCC(ee) Design Compared with LEP : Two beam pipes (larger no. of bunches 800 v/s 8 @Z), higher SR power (50 vs 0.33 MW), lower y*, s=230 GeV for ZH running (v. similar no. of Higgs bosons produced as at s=240 GeV but with 18% lower SR loss) Design of LEP3 follows closely that outlined in FCC MTR and [ESPP 2013: https://cds.cern.ch/record/1471486]. Separate full energy collider and accelerator (booster), the latter for top-up injection. Electrons and positrons in the collider ring travel in separate beam pipes. With top-up as beam lifetime is expected to be ~ 15 minutes (loss due radiative Bhabhas) top up ~ few 1010 electron/s Earlier work (2013) ESPP Venice Jun25 Note ratio of Lumi (1/5) 7

LEP3 Principal Technical Parameters Our List of Parameters Preliminary 2017 Optics design from Oide/Shatilov gives (for 4 expts at 240 GeV) s (GeV). 1034 cm-2s-1 240 1.1 91.2 52 4 2 Xpts: L/IP increases by ~ 1.15 240 230: L/IP increases by ~1.14 1.1 1.44 1034 cm-2s-1 However, we prefer to quote a LuminosityRange s (GeV). 1034 cm-2s-1 230 1 2.5 160 5.0 7.5 91.2 30 50 8 ESPP Venice Jun25

LEP3 Significant Parameters and the Running Sequence No. of IPs Can t reach t-tbar threshold Energy for ZH Fix SR power loss at 50 MW/beam SR energy loss/turn Total RF voltage installed ~ 6 GV Crossing angle For event numbers - use Inst.Lvalues of L /IP~ 1.6, 6.2, 40 (1034 cm-2s-1) at Z, WW, ZH One Running Scenario Est. total no. of Events 2 Long Shutdowns 230 GeV ~ 5.4 GeV 30 mrad 1yr 2yr 1yr 1yr e.g. 6 yrs at 230 GeV, 4 years around WW, 6 years around Z 4.2 x105 H, 3.7 x 107 WW at 157 and 163 GeV, 1.7 x 1012 Z at & around 91.2 GeV (cf. 109/ 6.1012Z for ILC/FCC-ee) LEP3 is competitive with alternatives/Plan B projects wrt Higgs and E-W physics Superior for WW and Z running Running time: 185 days of operation at 75% efficiency, with top-up running, giving 1.2 107 s/year of running 9 ESPP Venice Jun25

Higgs boson physics The errors evaluated for the determination of the Higgs boson couplings, tend to be dominantly statistical, which scale as the (ratio of event numbers) The event numbers are ~ 4 x105 H bosons for LEP3 and LC, and 22 x105 H bosons for FCC-ee (240 only) i.e. ratio of 1:1:~5.5 For equal power of the experiments error bars for LEP3 and LC will be a factor >2 larger than for FCC-ee. Detailed comparison will be presented by the ESPP Physics Group in the Briefing Book. Also see talks in Monday s EWK parallel session. 10 ESPP Venice Jun25

E-W physics ~1.7x1012 Z bosons (over 6 yrs), LEPZ proposal would give similar/same results and ~ 3.6 107 WW pairs (over 4 yrs at/around s=163 GeV) The event numbers are ~ 1.7 x1012 Z bosons for LEP3, 109 for LC, and 6 x1012 for FCC-ee at & around Z-pole i.e. in the ratio of ratio of 1:10-3:3.5 Search for HNL requires as large a Z-boson sample as possible. For equal power of the experiments LEP3 is superior to LC Detailed comparison will be made by the ESPP Physics Group. Also see talks in Monday s EWK parallel session. The comparison made here is taken from Z Factory Options for CERN , A. Blondel & M. Selvaggi https://doi.org/10.17181/0zn0c-3xe20. The systematic and statistical errors are mostly scaled by event numbers Z-pole WW sin2 W 11 ESPP Venice Jun25

Further Studies and R&D Follow FCC(ee) design for most components Dipole magnets, injector, collimation and vacuum systems, synchrotron energy loss absorbers etc. Differences 800MHz RF; ZH running at 115 GeV instead of 120 GeV, 2 experiments instead of 4, Injection energy of 10 GeV (instead of 20 GeV), We need further studies on the following items: Developing LEP3Lattice, IR design Crossing angle and its implications Integration studies (e.g. RF system, of power sources in klystron galleries .) Radioprotection and LHC dismantling We would also require R&D on RF cavities and fundamental power couplers, superconducting quadrupoles and sextupoles to reduce power consumption and improve fill factor 12 ESPP Venice Jun25

RF System: Current Recommendation of CERN RF Group Key determining drivers: beam current, power, acceleration gradient RF: Energy loss/turn @ s=230 GeV: 5.4 GeV 6.0 GV has to be installed, keeping the same margin as FCCee. Installed in even number LSSs (~560m, = 4.4m) (two LSSs for the collider, two LSSs for the booster) As overall cost is dominated by the cost of the RF system (~1.5 BCHF), Current thoughts from CERN RF Group; deploy 800MHz bulk Nb cavities run at 2K, two types (1-cell and 4-cells) for the collider and one for the booster (6-cells), all inspired by FCC designs. ALL CRMs costed as FCC-ee 6-cell cavities. For Z running: 800MHz 1-cell bulk niobium cavities. Keep existing QRL. The beam current is too high to use 800 MHz multi-cell cavities. Separate cavities for electrons and positrons. Reuse the power couplers (a costly item) for WW and ZH running For WW and ZH running: 800 MHz 4-cell sc bulk niobium cavities. The two colliding beams share RF at ZH energy. For Booster: 800 MHz 6-cell sc bulk niobium cavities. 13

Choice of Components: RF System Current Thoughts Integration of FCC-ee 800 MHz Cryomodule into LEP/LHC tunnel e.g. Point 4 14 ESPP Venice Jun25

Civil Engineering 40 years old infrastructure, some parts need maintenance High lumi achieved by crab-waist large xing-angle need to widen the LSS cross section on either side of the experiments LEP3 should have a low environmental impact booster by-pass (possibly not needed) Overall civil engineering cost estimated: 165 MCHF 15 ESPP Venice Jun25

Cost Use the FCCee costing methodology. Costs from FCC(ee) MTR : scaled according to numbers of components required. RF costs from CERN-RF Group. Cost: assumes careful removal of LHC machine in case it is needed later as an injector. We believe that LEP3 will allow pursuit of other future-oriented initiatives [e.g. R&D on RF cavities (high gradient, low power), RF fundamental power couplers, nested sc quads/sextupoles, high-field magnets, muon collider demonstrator etc.] Sustainability is an important issue: LEP3 should have a much lower environmental impact than a new flagship project but can still use ideas/proposals from the other projects 16 ESPP Venice Jun25

Example Shutdown Schedule: After LHC stops 17 ESPP Venice Jun25

Summary We assume that FCC-ee and FCC-hh will be the preferred option (justifiably so) for CERN s future We believe that the preferred scenario is technically feasible. If it is deemed to be not feasible financially, then the intermediate solution en-route to the next collider that will probe ~10 TeV must be significantly less costly than the FCC-ee. LEP3 is a reasonable (perhaps the best) backup option, Compared to the linear e+e colliders proposed, LEP3 provides similar luminosity for ZH production, higher luminosity at lower energies and options for multiple experiments, all at a much lower cost. Leaves room (time, budget, resources) for further development of THE machine that can probe directly the energy frontier at a ~ 10 times LHC. A real strategy for Europe, when resorting to such a plan B MUST include a long-term plan beyond LEP3, as well. No showstoppers have yet been identified, and we consider this proposal to be sufficiently interesting to deserve further study. We have identified important areas that would require deeper investigation before CERN could commit to LEP3. 18 ESPP Venice Jun25